American Express is officially selling its massive Costco business to Citi

Last February, American Express announced the end of the exclusive co-branding agreement on grounds that it couldn't find a deal that made better economic sense. The end of the deal was slated for this March.

In a statement Monday, American Express confirmed that it has sold the portfolio to Citi. The sale is expected to close in June, and all Costco customers with co-branded American Express cards will be transferred to Citi.

American Express could earn up to $1 billion from the sale, according to the statement.

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American Express is officially selling its massive Costco business to Citi

An employee pulls a cart full of tires past the cafe at a Costco Wholesale Corp. store in Nashville, Tennessee, U.S., on Friday, Sept. 25, 2015. Costco Wholesale Corp., the largest U.S. warehouse-club chain, is expected to release fourth-quarter earnings figures on September 29. Photographer: Luke Sharrett/Bloomberg via Getty Images

Employee Jordan Meinders sorts a display of books at a Costco Wholesale Corp. store in East Peoria, Illinois, U.S., on Tuesday, May 26, 2015. Costco Wholesale Corp., the largest U.S. warehouse-club chain, is expected to release third-quarter earnings figures on May 27. Photographer: Daniel Acker/Bloomberg via Getty Images

An employee pushes a row of shopping carts into a Costco Wholesale Corp. store in East Peoria, Illinois, U.S., on Tuesday, May 26, 2015. Costco Wholesale Corp., the largest U.S. warehouse-club chain, is expected to release third-quarter earnings figures on May 27. Photographer: Daniel Acker/Bloomberg via Getty Images

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The loss of the exclusive deal with Costco was seen as a huge blow to American Express, which also recently lost an agreement with JetBlue, and faced an antitrust lawsuit from the Department of Justice.

And in a CNBC interview Monday, Warren Buffett, the company's largest shareholder, said the loss of Costco was a "big deal".

He spoke about American Express after a viewer question that wondered why Berkshire Hathaway is still invested in the company even after all its recent issues.

Buffett noted that it's a very strong brand with a great reputation that will continue to face lots of competition.

American Express shares were little changed in pre-market trading.

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American Express is officially selling its massive Costco business to Citi